This invention relates to an apparatus and method of performing x-ray diffraction analysis. In particular, this invention relates to performing x-ray diffraction on large (over 12 cm in diameter) parts or in assembly line environments. This is accomplished by decoupling sample movement from the movement of a detector and x-ray source by replacing the goniometer base used in conventional x-ray diffraction instruments. The goniometer base is replaced with holders (robots) which move the x-ray source and/or detector independently such that the x-ray source or detector are not physically attached to the sample.
In addition to eliminating the inherent restrictions placed on the sample size by the prior art goniometer base, restrictions on reciprocal space coverage are also reduced. Conventional goniometer bases have only a fixed number of axes upon which the x-ray source, sample and detector move only in fixed planes. Incorporating robots to hold the detectors, x-ray source and/or sample allows for unrestricted motion anywhere in the robot's accessible envelope of reach. This also eliminates the need for specialty sample holders and attachments to provide additional range of motion for the sample.
By way of further background and explanation, over the years, a specialized field of materials analysis has developed in which x-rays are diffracted off of sample materials. Every crystalline material has a unique composition formed of regular, periodic spacings of electron densities. These are often referred to as "d-spacings" or "lattice spacings". Each compound has its own set of lattice spacings. Knowledge of these spacings is utilized to determine the compounds of which a particular material is composed. Additionally, it has been determined that stress in a material causes lattice spacings to expand or contract, as a result of compressive and tensile stress. If the composition of the material is known, the strain in a material can be measured by measuring the lattice spacing changes from the unstressed state. Further, if the grains in a material have a preferred orientation, rather than random orientation, variations in the intensity of the diffracted x-ray beam in certain directions will correspond to the grain orientation for that set of d-spacings. As a result, the orientation, sometimes called texture, of a material can be measured. The field of x-ray diffraction also enables measurement of other material properties such as crystallite size.
Operationally, when incident beam x-rays are diffracted from a material, they are diffracted in three dimensions, not in a plane. That is, when an incident x-ray beam hits a material, the diffracted x-rays actually form three-dimensional diffraction cones. Since each material is composed of many sets of d-spacings corresponding to the specific material being investigated, numerous diffraction cones are formed for any given material. Additionally, these diffraction cones have different diameters.
If the sample material is perfectly random, i.e., it has no preferred orientation, it is possible to obtain necessary information from any area on the diffraction cone. For randomly oriented materials, therefore, it is not necessary to move the detector and sample out of a given plane in order to observe and obtain the necessary information from various locations within the diffraction cone. To date, x-ray diffraction has primarily been performed on randomly oriented powdered materials in order to determine composition or on perfect, single crystals, in order to obtain a structure solution. As in most specialized fields, however, traditional applications are being extended to new methods and measurements. In the specialized field of x-ray diffraction, this is also true. Most sample materials of practical interest today do have some degree of preferred orientation. For example, semiconductor wafers, sputtering targets used to create those wafers, metals used to make aircraft or turbine blades, etc., all have some degree of preferred orientation. As x-ray diffraction has advanced to measure a material's properties, such as orientation and stress, it has become increasingly important and necessary to be able to access the entire diffraction cone.
Prior art response to this need has been the creation of two-dimensional (area) detectors, rather than a point or linear detector, so as to enable simultaneous coverage of a larger section of the diffraction cone. Further, it is known in the art to provide special sample holder attachments, often called circles or cradles, capable of moving the sample in an additional plane(s). By means of adding additional circles/cradles, in other words, it is possible to accomplish moving the sample to an increased range of positions by combining motions on numerous, independent axes moving in a circular plane. The primary system for these prior art x-ray diffraction analysis tools is a goniometer. On conventional goniometers, each independent movement is usually called an "axis". Most goniometers capable of collecting data on oriented sample materials have at least four independent axes (each moving in a plane and in a complete or partial circle) capable of positioning the sample in a given location relative to the incident x-ray beam. Further, the sample, detector and x-ray source are attached to and moved by the goniometer.
This solution to the problem is cumbersome, overly complex, and impractical to scale up, and critically, it restricts the sample size, weight and shape, since the sample must be held and moved by the goniometer base in the center of the goniometer circles (axes), all of which have fixed diameters. In this sense, the x-ray source, detector and sample are coupled. Because one of these circles, in the prior art, is holding and moving the detector and also holding, and possibly moving, the x-ray source, it is impossible to automatically vary either the x-ray source to sample distance or the sample to detector distance. In other words, in prior art devices, these distances are usually fixed on a given instrument. In one case known to the inventors, the sample to the detector distance is variable, but only manually and within a particular plane. In the SIEMENS brand area detector system devices, the detector is able to manually slide in or out on a fixed length arm attached to the horizontal plane circular axis. The detector still moves in a plane about the sample however, and can only be positioned at given distances (e.g., 6 cm, 15 cm, 20cm, or 30 cm) from the sample. Also, each time the detector is manually moved on the arm, new configuration parameters must be loaded, and the detector must be recalibrated.
Thus, there is a need in the art for providing an unrestricted motion apparatus and method for x-ray diffraction analysis enabling unrestricted coverage of the complete diffraction cone, which is capable of handling large, irregularly shaped samples, which eliminates the need for a variety of specialty sample holder "circles", and which allows the researcher to set up his or her own experiment without restriction. It, therefore, is an object of this invention to provide an unrestricted motion apparatus and method which provides for out-of-plane motion of the detector and/or x-ray source and simply and efficiently enables unrestricted coverage of the entire diffraction cone. A further object of the invention is to enable handling of large, at least over 12 cm in diameter, samples which may or may not be flat, and to enable the user to program his/her own unique data collection schemes.